Yarn supply apparatuses in knitting machines have the task of supplying the corresponding knitting stations with yarn of the requisite tension and in the desired quantity at the correct time in each case. The constancy of the yarn tension substantially determines the uniformity of the knitted product produced.
Fluctuations in the tension of the supplied yarn, especially when they recur systematically in one row of loops after another, can cause a marked impairment of quality of the resultant knitted goods. Fluctuations in yarn tension can occur when the yarn demand abruptly changes over time. This is the case for instance in flatbed knitting machines when at the turning point of the yarn guide there is suddenly no yarn consumption. If the yarn tension then fluctuates, the result is different loop widths at the edges of the resultant knitted goods than in the middle.
Especially with hard yarns, because of the nonexistent capability of the yarn to expand, the yarn tension depends on the yarn supply quantity, which should match yarn consumption as closely as possible at the particular current time.
For applications with yarn consumption that fluctuates over time, the yarn supply apparatus known from German Patent DE 36 27 731 C1 was developed; it has a yarn wheel driven by a stepping motor. The yarn wheel carries the yarn, drawn from a yarn bobbin, to the applicable knitting station via a yarn brake. The yarn supplied by the yarn wheel travels through a terminal eyelet of a lever supported pivotably on its other end. The eyelet represents a turning point, at which the yarn is rerouted at an acute angle. To adjust a constant yarn tension, the pivot lever is acted upon by a constant torque by means of a direct current motor. The pivot lever is also connected to a position transducer, which detects its pivoted position and readjusts the stepping motor accordingly. The pivot lever, in cooperation with the sensor device, thus serves to detect the existing yarn supply.
A closed-loop controller compares the position of the pivot lever with a command value and accelerates or decelerates the motor if the command value is exceeded or undershot. To compensate for sudden changes in demand, which the motor cannot follow instantaneously because of its moment of inertia, the pivot lever forms a yarn store, which can temporarily store a limited length of yarn.
On sudden changes in yarn demand, the pivot lever must be speeded up. The moment of mass inertia of the pivot lever has an effect on the yarn tension and impairs the constancy thereof.
From German Patent DE 38 20 618 C2, the yarn supply apparatus for kinky and other effect yarns is known, which has two rotationally driven yarn wheels, rotating in opposite directions, around which the yarn to be supplied is wrapped multiple times in a figure eight. An arm carrying an eyelet on its end and acted upon by torque in a predetermined direction of rotation acts as a yarn store for temporarily storing yarn intermittently not drawn off by the knitting stations. The yarn travels at an acute angle through its terminal eyelet, and for temporary storage it is deposited on bolts or posts located along a circle around the arm.
Frictional effects that affect yarn travel occur both on the bolts or posts forming a temporary store and at the eyelet of the arm through which the yarn travels at an acute angle.
From German Patent Disclosure DE 42 06 607 A1, a yarn supply apparatus for simultaneously supplying two yarns to a knitting machine is known, in which a yarn supply wheel is driven by a disk rotor motor. At least one yarn travels from the yarn supply wheel through the longitudinal opening of a helical spring wound in a conical or trumpet shape. A permanent magnet and a Hall sensor are provided on a bearing that pivotably holds the helical spring on one end, to enable detecting deflections of the helical spring. On the basis of these deflections, the disk rotor motor is readjusted, so that the command length of the helical spring is established in steady-state operation. In that position, the yarn travels laterally along the inner wall of the helical spring, through the opening in it. The helical spring acts as a spring and damping element, which allows a certain temporary storage of supplied yarn.
Yarn supplied because of the moment of inertia of the disk rotor motor is received by the temporary store, which changes the yarn tension.
Finally, U.S. Pat. No. 3,858,416 discloses a yarn supply apparatus which is suitable for knitting machines that have substantially constant yarn consumption and for supplying hard yarns. The yarn supply apparatus has an electric motor whose rpm is controllable via the applied voltage and which by means of a suitable yarn wheel draws yarn from a bobbin and delivers it to the appropriate knitting station via a yarn tension sensor. A command value transducer is also present, which is connected to a command value input of a closed-loop controller, via a reversing switch and via selectively actuatable adjusting devices. Via the reversing switch, the controller receives a signal, characterizing the yarn tension, at its actual value input, and it readjusts the motor accordingly. Rpm sensors are also present on the electric motor and on the knitting machine; given a suitably different switch position of the reversing switch, they can be connected to the command value and actual value inputs of the controller. The reversing switch allows a switchover from one operating mode, with a yarn tension regulated so that it is constant, to an operating mode with a defined yarn supply quantity. Each knitting station of the circular knitting machine is assigned a corresponding yarn supply apparatus; so that the quantity of yarn to be supplied corresponds to the yarn consumption of a knitting station. The yarn travel speed is correspondingly low.
There are no provisions made for temporarily storing any possible excess lengths of yarn supplied as a result of motor inertia or motor characteristics or suddenly required to be paid out. Sudden changes in yarn demand, because of the reaction time of the controller and of the connected motor, therefore cause yarn tension spikes, which in an extreme case can lead to yarn breakage.